Storm Day, Start to Finish · MesoRadar Learn

In short: a severe weather day has an order. Morning: the outlook, the balloon soundings, and what last night’s storms left behind. Late morning: the boundaries on the surface map, because that is where storms start. Early afternoon: the cap weakening and the first towers. Storm time: careful radar and every warning acted on. Evening: keep watching, because the wind changes near sunset and lines can run all night. Each step links to the guide that explains it.

1
Surface low. The fronts hang off it; winds turn around it counterclockwise.
2
Cold front. Triangles point the way it moves. Strong lift, often a fast-growing line of storms.
3
Warm front. Half circles point the way it moves. Storms near it can tap extra low-level spin.
4
Dryline. Moist air to the east, desert air to the west. Storms go up on the moist side.
5
Triple point. Where the dryline meets the cold front. A common first target.
6
Warm sector. Warm, moist air on southerly wind: the fuel.
7
Old outflow boundary. Rain-cooled air left by last night's storms, still on the map at noon.
8
Boundary crossing. Where the outflow meets the dryline, the two sources of lift add together.

A classic spring setup on the Plains, simplified. Real days rarely look this tidy: the dryline bulges, fronts stall, and the outflow boundary may be the only one that matters.

The night before and first thing

The evening before, read the Day 2 outlook; the first Day 1 outlook for the day comes out at 0600 UTC (1 a.m. CDT).

In the morning, start with the SPC Day 1 Convective Outlook. It names the risk category, from general thunderstorms up to High, and gives separate chances of tornadoes, damaging wind and large hail within 25 miles of a point. SPC issues it at 0600, 1300, 1630, 2000 and 0100 UTC (1 a.m., 8 a.m., 11:30 a.m., 3 p.m. and 8 p.m. CDT). Read the text, not just the colors: the forecaster says which storm mode is expected, when, and what could go wrong. Check the issuance time on spc.noaa.gov before trusting any copy. What each category means.

Next, the 12Z soundings (Z means UTC, so 12Z is 12 UTC, 7 a.m. CDT). The balloons go up about an hour earlier, near 6 a.m. Open the one nearest your target and compare it with the model at the same place for the nearest hour it offers. If the model is too moist near the ground, or its cap too weak, carry that error forward into its afternoon. Reading an observed sounding.

Then ask what happened overnight. Storms before dawn leave rain-cooled air behind, and its leading edge, the outflow boundary, can sit on the map until afternoon, along with clouds that slow the warming. A messy night can ruin a day or set up its best target.

In MesoRadar: Outlooks, Severe outlook, with the Day 1 Risk chip (free) and the Tornado, Hail and Wind chips (Pro). The Skew-T tool (Pro) in the map tools: tap an amber dot for the observed balloon sounding, or tap anywhere for a HRRR sounding, and step through the hours with the hour chips.

Late morning: find the boundaries

Storms need moisture, instability and lift. By late morning the first two are usually set; the question is where the lift will be. It comes mostly from boundaries, lines where two kinds of air meet and push into each other. On the surface observations, look for sharp changes in temperature, dewpoint or wind direction over a short distance.

Fronts

A cold front pushes cooler, denser air under warm air. The lift is strong, so storms on it often merge into a line quickly. A warm front is warm air advancing over cooler air; a storm riding along it or just on its cool side can find winds that turn more sharply with height, so warm fronts get close attention on tornado days. A front that stalls is a stationary front, and storms can train along it, raising the flood risk.

Dryline

The dryline separates moist air from the Gulf of Mexico from dry air off the deserts. It is mostly a High Plains feature of spring and early summer. Dewpoints can drop 20 °F (11 °C) or more across a few miles while the temperature barely changes. On a typical day it moves east through the afternoon as the moist layer mixes out from the west, then backs west after dark. Storms form on its moist (east) side, often at a bulge, and where it meets the cold front, the triple point.

Outflow boundaries

An outflow boundary is the edge of rain-cooled air from earlier storms. It acts like a small cold front: a wind shift, cooler air behind it, lift along it. NOAA notes that one can last a day or more after its storms die and travel hundreds of miles. To find one:

Surface obs: a line of stations with cooler air and winds blowing away from the old storms.
Visible satellite: a curved line of small cumulus at its edge, or clear air behind it where the cool air suppresses clouds.
Radar: a thin fine line of weak echo, often 5 to 20 dBZ, made by insects and dust caught in the lift.

Boundaries matter for tornadoes as well as for starting storms. In the VORTEX-95 field project, close to 70% of significant tornadoes happened near a low-level boundary, most on its cool side and within about 30 km (Markowski, Rasmussen and Straka 1998). A supercell crossing a boundary is one to watch.

In MesoRadar: Surface observations in Layers, then tap a station for its temperature, dew point and wind. The National tab's surface analysis rows, such as Dewpoint (2m), map the moisture edge. Visible (Red) on the Satellite tab, and how to read visible satellite.

The cap and how it breaks

The cap is a layer of warm air aloft that stops surface air from rising. It is why a day with huge CAPE can stay sunny. Storms start when lift along a boundary, heating, or cooling aloft wears it down. How the cap looks on a sounding.

Three checks during the day:

CIN trend. Watch the model’s convective inhibition shrink through the afternoon hours where the boundaries will be. What CIN measures.
Convective temperature. A sounding can give the surface temperature at which a parcel breaks the cap without help. If thermometers are well short of it by mid-afternoon, a boundary has to do the lifting.
The sky. Towers that keep building in one place and harden into firm, cauliflower tops are a cap giving way. Towers that go up and shear apart are a cap holding.

Early afternoon: discussions, watches, first towers

A mesoscale discussion is SPC’s note on what is developing over the next few hours. On severe days it often says whether a watch is likely, and SPC aims to issue one an hour or two before a watch. Then comes the watch itself: conditions are favorable, so be ready. Watches versus warnings.

This is also the time to judge storm mode. The supercell composites (SCP and STP) assume storms stay separate. When the deep-layer shear and the mean wind through the cloud layer point along the boundary, storms stay on it and tend to merge into a line within about 3 hours; when they cross it at a large angle, storms move off it and stay separate longer. Strong convergence on the boundary speeds the merge (Dial, Racy and Thompson 2010). Will storms stay separate? Check the shear and helicity themselves too: shear, helicity, the hodograph and the composites, each with its caveats.

In MesoRadar: Outlooks, Discussions, Mesoscale discussions (free); Meso discussions in Layers draws their areas, and Alerts in Layers shows NWS watches and warnings. On the National tab, the HRRR groups (Pro): Instability for CAPE and CIN; Shear & Rotation for Bulk Shear 0-6 km, Storm-Relative Helicity and Updraft Helicity 2-5 km; Composite Indices for Significant Tornado Parameter and Supercell Composite.

Storm time: radar

Check the frame time first. Radar reaches any app a few minutes after the scan, and a storm at 50 mph covers four miles in five minutes. What radar cannot show you.

Then decide which storms matter:

The storm with clear air around it. A discrete storm at the south end of a line, or the first cell off the dryline, has the inflow to itself. Storms in a cluster compete for the same air.
The storm on a boundary. A supercell moving along or across a front or outflow boundary.
Structure. A hook, an inflow notch and a tight velocity couplet together point to strong rotation. Supercells, hook echoes, lines and bows.
Velocity, storm-relative. Subtracting the storms’ motion makes rotation stand out. Doppler velocity. Big cores aloft point to hail. Hail on radar.

A couplet on radar means strong rotation. Only the NWS issues the warning. Most tornado warnings are based on radar and mean a tornado is occurring or could form at any moment, so act on every one; a tornado on the ground is confirmed by a spotter or public report, a warning marked OBSERVED, or a damage survey.

In MesoRadar: Storm-Relative Velocity in the Local tab's Radar Product picker. The National tab's Rotation row, Low 0-2 km or Mid 3-6 km, with a 1 hr, 6 hr or 24 hr window, maps rotation tracks (Pro). Live lightning in Layers (free), and what lightning tells you. How the timeline works.

1
Morning balloons. Released around 6 a.m. for the 12 UTC sounding.
2
1300 UTC outlook. 8 a.m. The first full read of the day.
3
1630 UTC outlook. 11:30 a.m. Often the one that sharpens the target.
4
2000 UTC outlook. 3 p.m. Updated with what storms are doing.
5
0100 UTC outlook. 8 p.m. Covers the night.
6
Discussions and watches. Typically late morning through mid-afternoon.
7
Storm time. Most often mid-afternoon to evening.
8
Evening window. Around and after sunset, as the low-level jet strengthens.

A typical Plains storm day in Central Daylight Time, with ticks every three hours from 6 a.m. Outlook times are fixed; everything else shifts from day to day, and storms can start at any hour.

Evening: the jet, the line, the water

Near sunset the ground cools and the wind just above it often speeds up into the low-level jet. That can raise low-level shear around storms already going. In simulations of two VORTEX2 storms, adding the evening rise in low-level shear made their low-level updrafts stronger and better at lifting cool air near the ground (Coffer and Parker 2015). That is a reason to keep watching after dark, not a promise of tornadoes: on many evenings the cooling wins and storms weaken.

Storms often grow into a line or a cluster by evening and run through the night, with damaging wind and spin-ups that are hard to see. Slow or training storms bring the night’s biggest risk for many people: flash flooding. Flash flooding.

In MesoRadar: the Rain outlook in Outlooks for the flood risk. Storm reports in Layers for what spotters and the public reported.

Safety

The NWS is the authority here; these are its main points.

When a warning covers you, shelter. Lowest floor, interior room, away from windows, head covered. A mobile or manufactured home is not safe in a tornado: leave for a sturdy building before the storm arrives.
Have more than one way to get warnings. Phone alerts, a NOAA Weather Radio, local media. Phones get Wireless Emergency Alerts for tornado warnings and the most dangerous severe thunderstorm and flash flood warnings, not every warning.
Never drive into the rain wrapped around a rotating storm or into a hail core. The circulation, large hail and flooded roads can all be inside it, out of sight.
If you are out watching storms, keep more than one escape route. Know the roads ahead of time, stay out of the storm’s path, and leave early; a road you planned to use can close behind you.
Never shelter under a highway overpass. Wind speeds up through it and debris flies through it. Drive to a sturdy building if you can.
Turn around at water over a road. More than half of flood drownings happen in vehicles driven into water.
When thunder roars, go indoors or into a hard-topped vehicle with the windows up, and wait 30 minutes after the last thunder.
If you spot, report. SKYWARN training teaches what to report and how.

The checklist

Day 1 outlook: category, hazard chances and the text. Note the issuance time.
12Z sounding nearest your target, against the model’s nearest hour.
Overnight storms: where did their outflow end up?
Surface map: fronts, dryline, outflow boundaries, where the moisture is.
The cap: CIN trend, convective temperature, towers that keep building.
Mesoscale discussions and watches.
Storm mode: shear and storm motion against the boundary.
Radar: frame time first, then the storm with clear air or on a boundary.
Evening: keep watching; then the line, then the water.
Warnings: act on every one.

Key points

The SPC Day 1 outlook updates at 0600, 1300, 1630, 2000 and 0100 UTC; read the text and check the time
Storms start on boundaries: fronts, the dryline, old outflow, and where they cross
The cap decides whether storms form; watch CIN and the towers
Shear and mean wind along the boundary favor a line; across it, separate storms
Check the radar frame time first; a couplet means strong rotation, not a confirmed tornado
Have more than one way to get warnings, and never drive into a storm’s core

Common questions

When does SPC update the Day 1 outlook?

Five times a day: 0600, 1300, 1630, 2000 and 0100 UTC. In Central Daylight Time that is 1 a.m., 8 a.m., 11:30 a.m., 3 p.m. and 8 p.m. Check the current issuance time on spc.noaa.gov, since a special update can also be sent when the forecast changes.

What is a triple point?

The spot where three boundaries meet, most often where the dryline meets the cold front, close to the surface low. Air converges from several sides there, so it is a favored place for the first storms of the day, though not the only one.

Why do storms start on boundaries?

A boundary is a line where two kinds of air meet, and the winds on either side push into each other. That forces air upward. If the air is unstable and the lift is enough to break the cap, storms form along the line, often at a bulge, a kink or where two boundaries cross.

Are evening tornadoes more likely than afternoon ones?

Not as a rule. Near sunset the low-level winds often strengthen while the ground cools, which can raise low-level shear around storms that are already going. On some days that tips the balance toward tornadoes; on others, cooling near the ground chokes storms off. Treat it as a reason to keep watching, not as a forecast.